Sudden Cardiac Arrest (SCA) is one of the major reasons of death in industrialised countries. The condition is characterized by life-threatening abnormal rhythms of the heart, so called arrhythmia. Common arrhythmia includes Ventricular Fibrillation (VF) or Ventricular Tachycardia (VT)—a quivering of the heart that impairs its function of pumping blood to the body and brain. The patient loses its pulse, eventually consciousness and finally breathing ability. All these symptoms can occur in a matter of seconds. Life support and rescue needs to be fast.
In supporting the rescue of patients, Automated External Defibrillators (AED) play a major role. An effective treatment is the delivery of an electrical shock to defibrillate the heart. For this to be successful, it is necessary to recognise the presence and indication for a shockable rhythm. To keep the reduced blood flow in the meantime from damaging the patient's organs and brains, Cardio Pulmonary Resuscitation or Reanimation (CPR) is applied. Also, some type of arrhythmia can be treated by the first respondent with CPR alone, while waiting or the emergency team.
An essential feature of AED is thus the capability to detect shockable rhythms. The VF detection algorithms have to advise to shock with an accuracy of more than 90% (sensitivity) and advise not to shock with an accuracy of more than 95% (specificity). For AED that are to be used in the public, eventually by untrained personnel, it is thus critical that safety for the patient and rescuer is warranted. The American Heart Association has set forth recommendation for Specifying and Reporting Arrhythmia Analysis Algorithms Performance (AHA Scientific Statement, Kerber, R. E. et. al., Circulation Vol. 95, No 6, Mar. 18, 1997).
The benchmarks above are defined on the basis of non noisy signals. The main kind of noise that could impair the analysis process, are artefacts generated by the chest compression on the thorax of the patient. These artefacts are highly non-reproducible from patient to patient or from rescuer to rescuer. The pattern might even change for a specific rescuer over time.
Detection of a shockable rhythm is done by analysing the patient's Electrocardiogram (ECG). The ECG reproduces the activity of a patient's heart by graphically displaying the electrical activity of the heart. AED commonly use sophisticated algorithms for analysing the patient's heart rhythm and devising the therapy, e.g. indicating the presence of a shockable rhythm. One common problem is the presence of artefact signals that can result from various sources, the most prominent being the performance of CPR upon the patient. Artefacts can also result from patient motion during transport, the rescuer unintentionally touching the electrode pads, the patient's electrode—skin contact and many other sources.
The state of the art suggests varying ways for addressing the problem of artefacts. The presence of artefacts could lead to a wrong diagnosis. Thus, U.S. 2006/0025825 discloses a way to minimize the risk of a wrong diagnosis because of artefacts. For this end transthoracic impedance is measured separately from ECG. Based on the presence or absence of transthoracic impedance variations it is decided whether to shock, not to shock or to halt ECG measurement as a whole. Transthoracic impedance measurement has long since been suggested as flanking method for reliability surveillance of ECG. It has been sought to increase specificity by using transthoracic impedance as indication of bloodflow, or heamodynamics (Johnston, P. W. et al, The transthoracic impedance cardiogram is a potential heamodynamic sensor for an automated external defibrillator, European Heart Journal (1998) 19, 1876-188).
There are also possibilities of compensating the artefacts during CPR. U.S. Pat. No. 6,287,328 shows a method of enhancing the detection and taking into account of artefact signals. Artefact reference signals (so called “non-event signals”) are measured along so called “event signals” (ECG signals) and correlated by multi-variable artefact assessment. Presence and significance of artefacts from multiple potential sources are detected along a cardiac event signal. This additional information is used to support the decision whether the signal of interest (for example ECG signals) is to be trusted and can be used for defibrillation shock decision.
A further way of minimizing signal disturbances during CPR is disclosed in U.S. Pat. No. 6,807,442. A measurement of the compression and/or inflation of the chest is correlated with the signal disturbance (eq. artefact). A filter is used to eliminate said disturbances. The algorithm can thus conclude while CPR still being performed, reducing the time used for shock delivery and allowing more time for CPR.
In the attempt of minimizing the time for reaching a decision to deliver a shock, U.S. 2007/0213775 A1 (included herein by reference) teaches a defibrillator with minimal delay following the CPR intervals. The delay is minimized by quickly discriminating the end of a CPR period.
U.S. 2006/0129190 A1 also suggests a method for rapidly delivering a defibrillation shock, if indicated, by determining a probability, based on a first set of signals for presence of a shockable rhythm. A precharging is initiated, if the probability is high enough and then, a second set of data determines whether the therapy can be delivered. This earlier charging should reduce the time lag caused by the capacitor having to charge.
All the prior art solution are insufficient though, as they do not duly consider the required sensitivity and specificity benchmarks when using artefact compensation, or require too much time for finding a shockable rhythm if they do not use an artefact compensation. If the time the ECG needs to find a shockable rhythm is essentially ‘hands-off’ time with no CPR being performed, there are detrimental effects for the patient. Survival rates are highest, when defibrillation is conducted within the first few minutes after onset of arrhythmia and “hands off” time is minimised.